Magnetic plug detector

ABSTRACT

A magnetic anomaly detector at a selected location on a pipe string detects the passing of a pig carrying a permanent magnet. To distinguish between a number of pigs, the magnets are differently oriented, in terms of which magnetic pole passes the detector first. To distinguish between a number of magnet carrying pigs, the magnets are oriented such that a planned series of field changes occur in preferred order for each pig passing the detector.

[0001] This invention pertains to means to detect the movement of aplug, or pig, moving along a pipe bore. Additionally, it pertains todistinguishing between different plugs passing a reference point bysensing the different nature of the magnetic characteristics of theplugs.

BACKGROUND

[0002] In the art, and as interpreted herein, pipeline plugs movevertically and pipeline pigs move horizontally, but in general they arephysically identical. The term pig and plug will be used interchangablyherein.

[0003] Most petroleum well drilling activity involves casing stringscemented in place in the earth. The cement is often moved, as a slurry,down the well in the bore of a pipe string. Often, a working pipe stringis used to extend an installation pipe string down to the location forcementing. After cementing, the working string is detached from theinstalled string and recovered. It is important that the cement goeswhere intended and nowhere else. Flow control is essential when pumpingcement down pipe string bores.

[0004] When pipe strings are cemented into wells, the well and the pipestring suspended down the well are initially full of fluid, usuallydrilling fluid. A plug, usually of resilient material, is installed inthe bore of the suspended pipe string and cement is pumped into thebore, pushing the plug and the initial fluid ahead of the advancingcement slurry. When the required amount of cement slurry has been pumpedinto the pipe bore, another plug is installed in the pipe bore. Adifferent fluid, a driving or displacement fluid, is then pumped intothe pipe bore to push the second plug, and the cement charge, along thepipe bore.

[0005] When the second plug gets to the bottom of the well, the cementhas been pushed upward along the annulus between the new pipe string andthe earthen wall of the well bore.

[0006] If either plug fails to move into and along the pipe string boreat the appropriate time, a bad cementing job can result. There is no wayto see the plug inside the pipe.

[0007] There is a need for a reliable, and reasonably simple, system fordetecting the movement of the plugs past critical points in the pipearrangement.

SUMMARY OF INVENTION

[0008] It is a purpose of this invention to provide means to detectspecially prepared plugs moving in pipe string bores.

[0009] It is another object of this invention to provide plugs, or pigs,with magnets and to provide stationary means to detect such pigs movingin pipe bores.

[0010] It is yet another object of the invention to provide pigs withdifferent magnet in polarity distributions and to provide means todistinguish between such different plugs.

[0011] These and other objects, advantages, and features of thisinvention will be apparent to those skilled in the art from aconsideration of this specification, including the attached claims andappended drawings.

[0012] A special plug, or pig, is prepared for movement along aconventional pipe bore. The plug is usually made of elastomericmaterial, with a rigid core of material that can readily be drilled upafter it serves the intended purpose.

[0013] To make the plug detectable it is fitted with one or moremagnets, either attached to the outside surfaces or imbedded in the plugmaterial. For small bore pipe with thick walls, the core of the plug,usually of non-metallic material is used to embed a magnet. For use withlarge plugs used in pipe with thinner walls, small magnets can beattached to the outside surface of the plug for successful and reliabledetection as it moves along a pipe bore.

[0014] To distinguish between a first and a second plug. The magnets aresimply oriented differently. To distinguish each plug from a largerplurality of plugs, combinations of magnetic orientations are employed.

[0015] The needed detection equipment already exists. The devicepreferred is used to detect the end of coiled tube moving along a pipebore. End detection is important if coiled tubing is being used toservice a well under pressure. If the tube is pulled out of all blowoutpreventers, a well is out of control. If the end of the tube isdetected, however, a lower blowout preventer can be closed to avoidproblems when the tubing is finally extracted and leaves the top blowoutpreventer open. In that application, a light weight steel tube isdetected within the bore of much heavier pipe.

[0016] Magnetic detection usually takes one of three forms; 1. detectionof a disturbance in an established magnetic field by the passage of thearticle being detected; 2. detection of a magnetic field caused by thepassage of the article being detected; and 3. change in the magneticpermeability, in the area selected for detection, as a result of thepresence of an article being detected.

[0017] To maximize the ability to detect the presence of substancescapable of influencing the magnetic field of an area, a magnetic fieldis often generated to permeate the area of interest. By that process,stray, or spurious magnetic fields have less influence. If the detectionsensitivity alone is amplified, movement of unrelated iron products inthe area can cause errors. Apparatus of this invention can utilize anyexisting detector system that is usable with the novel pigs but themagnetic field producing units are preferred.

[0018] Existing magnetic field characteristic detectors, when used withthe plugs of this invention, produce an output signal that moves anindicator first in one direction if a magnet passes the detection areanorth pole first. It first moves in the other direction if a plug ofthis invention passes the detection area south pole first. A differentreaction, providing the same selectivity, is accomplished by placing aring of bar magnets around the periphery of the plug, each magnet havinga particular pole directed radially outward. The second plug isdistinguishable if the magnets are oriented with the other polesdirected radially outward.

[0019] Combinations of magnet orientations provide detectablecharacteristics enabling the distinguishing of any plug from acollection of different plugs.

BRIEF DESCRIPTION OF DRAWINGS

[0020]FIGS. 1A and 1B are side views, coextensive, showing a verticalplane of earth, having a symbolic well with casing and a well pipestring suspended therein.

[0021]FIG. 2 is a side view, mostly in cut-away, of a detector stationon a pipe string, with a typical plug, or pig, passing along a pipebore.

[0022]FIG. 3 is a side view, mostly in cut-away, of a detector stationwith plugs passing in series, with plug distinguishing features.

[0023]FIG. 4 is similar to FIG. 3 with plugs of different distinguishingfeatures in position for series detection.

[0024]FIG. 5 is similar to FIG. 3 with plugs having furtherdistinguishing features in series in a pipe bore.

DETAILED DESCRIPTION OF DRAWINGS

[0025]FIG. 1 is a schematic presentation of a section of earthcontaining a well with surface casing SC installed and conducting a pipestring 9 into the well. Symbolic surface connections include two plugmagazines, 2 and 3, arranged to drop plugs by release actions of control5 and 6, on cue of the operators. Plug P1 is already dropped and haspreceded a cement column to the bottom of the well. The pipe string isfilled with cement which has pushed plug P1 out the bottom of the casingstring. Cement is flowing back up annulus 12. When the prescribed amountof cement has been pumped into the pipe extension 1 plug control 5 willbe activated to release plug P2 from magazine 3 into the pipe string. Aforcing fluid, usually drilling mud, will follow plug P2 until plug P2is pushed out the bottom of the casing string 9.

[0026] Excepting the use of the detector DET. of this invention, thisfigure could represent prior art.

[0027] This schematic is intended to be an oversimplification of theactual processes and machinery involved. It does provide background forthe points of novelty of this invention.

[0028]FIG. 2 shows a pipe 9 supporting sensor pack 13 wired by leads 15to indicator 16 to detect plug 11 carrying magnet 11 b in core 11 a asplug P3 passes the sensor while moving through the pipe bore. The plugcomprises radially extending elastomer mass 11 c configured to occludethe bore but pass through easily.

[0029] When one end of the magnet reaches the plug sensing members 13,the needle 16 a on indicator 16 first swings one direction, plus in thisinstant, then swings in the opposite direction when the positive polepasses the detector. If the plug remains the same otherwise, and themagnet is turned around to advance the north pole through the sensorregion first, the needle 16 a on the dial 16 first swings toward thenegative side before reversing movement.

[0030] That sequence of movements, and the differences between responseto different pigs passage, represents the signal characteristics thatcan be used to distinguish between pigs.

[0031]FIGS. 3, 4, and 5 are of similar layout, showing plugs in seriesmoving through the bore of pipe 9. The plug components differ and havedifferent captions. Plugs and pigs move in pipe bores with the extremeends of salient members lagging, due to dragging on the pipe bore wall.All plugs shown are moving or prepared to move downward.

[0032] The magnet core 22 of plug P4 in FIG. 3 moves South pole firstthrough the sensor field of detector element 13. Needle 16 a swingsfirst in one direction, then swings to the other direction as the Northpole moves through the detector field. When the plug P3, with core 21,moves through the detector field the needle will swing first in thedirection opposite the one experienced with core 22, and will thenswitch to the opposite side of zero as the trailing magnetic pole movesthrough the detector field.

[0033] The operation is simple and the indication clear enough fordirect observation to make it unnecessary to provide more sophisticatedinstrumentation. The signal that drives the dial readout shown, however,can be supplied to processing circuits to drive chart recorders, digitalrecorders, or other storage and indicator devices, if preferred. Asdefined herein an output signal from the detector is construed to be asignal adapted to the purpose. Such is well within the scope of thoseskilled in the art of instrumentation. That is anticipated by and iswithin the scope of the claims.

[0034] The plugs of FIG. 4 are distinguishable from either plug of FIG.3. Magnets 28 and 29 in one core lid yield a detector signal with adouble excursion as the plug moves through the detector. The plug P5,with core 11 e and magnets 23 and 24, with north poles outward causes aslowly rising south pole response, a quick north pole indication and atrailing off of a south pole indication. The four plugs of FIGS. 3 and 4can be distinguished if fed through the detector field in any order atreasonably similar velocities.

[0035]FIG. 5 shows a modified plug P7, with magnets 25 and 26 orientedto yield a more complex movement display of indicator 16 a. Core 11 f isusually a drillable or crushable material. Elastomer portion 11 k hasbeen modified by removal of a salient ring to make room for magnets 26.Magnets 25 are imbedded in the plug. Strap-on magnets such as 26 makepossible several combinations of magnetic characteristics to be used onplugs that are otherwise similar, and can be stocked on-site inquantity. Plug P8 is similar to plug P3.

[0036] From the foregoing, it will be seen that this invention is onewell adapted to attain all of the ends and objects hereinabove setforth, together with other advantages which are obvious and which areinherent to the apparatus.

[0037] It will be understood that certain features and sub-combinationsare of utility and may be employed without reference to other featuresand sub-combinations. This is contemplated by and is within the scope ofthe claims.

[0038] As many possible embodiments may be made of the apparatus of thisinvention without departing from the scope thereof, it is to beunderstood that all matter herein set forth or shown in the accompanyingdrawings is to be interpreted as illustrative and not in a limitingsense.

The invention having been described, we claim:
 1. A pipeline pigcharacteristic detecting apparatus for determining when a pig passes aselected location, and determining which pig, of a selection of pigs,passes the selected location, the apparatus comprising: a) a pipestring, along the bore of which pigs can pass; b) at least two pigs,having distinguishably different identifiable characteristics when thepigs are within the bore of the pipe string. c) a magnetic field changedetector means operable form outside the pipe string to indicate when apig carrying a selected array of magnets passes a selected location andto indicate which pig passed; d) an indicator means to output a signalwhen one of the pigs passes, the signal having a characteristicidentifiable with the identifiable characteristic of the pig.
 2. Theapparatus of claim 1 wherein said distinguishably different identifiablecharacteristics are provided by selective orientation of the poles ofmagnets carried by the pigs.
 3. The apparatus of claim 2 wherein atleast one said magnet is embedded in the material comprising the pig. 4.The apparatus of claim 2 wherein at least one of the magnets is securedto the outer surface of the pig.
 5. The apparatus of claim 1 whereinsaid distinguishably different identifiable characteristics is the pigsinfluence on an existing magnetic field.
 6. The apparatus of claim 1wherein said distinguishably different identifiable characteristicscomprises a magnetic field caused by said pig.
 7. The apparatus of claim1 wherein said distinguishably different identifiable characteristicscomprises the magnetic permeality nature of the pig.
 8. A pipeline pigcharacteristic detecting apparatus for determining when a pig passes aselected location, and determining which pig, of a selection of pigs,passes the selected location, the apparatus comprising: a) a pipestring, along the bore of which pigs can pass; b) a plurality of pigs,each pig having an axis of symmetry with a fore end and an aft end; c) aplurality of permanent magnets, at least one magnet carried by a firstof said pigs, the magnet arranged to proceed north pole forward when thefirst pig moves along a pipe bore; d) a second of said pigs arranged tocarry at least one of said magnets, the magnet arranged to proceed southpole forward when the second pig moves along a pipe bore; and e) amagnetic field change detector, having a detection range, situated torespond when one of said pigs moves through the detection range of thedetector, and to produce an output signal when a pig is detected.
 9. Theapparatus of claim 8 wherein said output signal indicates whether asouth pole or a north pole reaches the detection range of the detectorfirst.
 10. The apparatus of claim 8 wherein said signal indicates whichmagnet arrangement passed the detector whereby the pig situated withthat magnet arrangement can be distinguised from other pigs.
 11. Amethod for use with pigs moving along the bore of a pipe string toisolate selected bodies of fluid between at least two pigs distributedaxially along the bore and to ascertain which of the pigs pass aselected pig detecting region of the pipe string, according to thesteps: a) placing permanent magnets on the pigs to be used; b) orientingthe magnets on a first pig such that a selected magnetic pole will bethe first pole to pass a selected plane on the pipe string when thefirst pig is moving along the pipe bore; c) orienting the magnets on asecond pig such that the opposite magnetic pole, compared with saidfirst pig, will be the first pole to pass the selected plane when thesecond pig is moving along the pipe string bore; d) placing a detectorsuch that it can detect changes in the magnetic field in the vicinity ofthe detector when one of the pigs passes the detector, and produce anoutput signal bearing distinguishing information relative to which pigpassed the selected plane; and e) passing the pigs along the pipe stringbore.